Biochemical studies of the last several years have shown that microbial iron transport and mobilization take place using very powerful low-molecular-weight chelating agents which solubilize ferric ion by coordination. These compounds, called siderochromes, are manufactured by various microorganisms as needed to extract ferric ion from their solution environment and concentrate it within the cells. This process recently has been linked to the pathogenicity of microbial infections, so it is not surprising that several of the siderochromes are potent and broad-spectrum antibiotics, while others are growth factors. Transport of the siderochromes across cell membranes is in some cases very conformation dependent; the metal complexes are passed rapidly into the cell while the demetallo forms are not. We have replaced Fe 3 ion by Cr 3 ion in several of the siderochromes to make their isomers kinetically inert. Some isomers have been separated and characterized and we have shown that in at least one case the inert wedge-cis isomer of the Cr 3 ion complex is transported into the cell at the same rate as the native labile ferric complex. We propose to continue using Cr 3 ion substitution to characterize the coordination geometries of the siderochrome complexes, use the separated isomers in transport studies in several microbial systems and determine the structures by X-ray diffraction of selected members of the various types of siderochromes.